It has heretofore been recognized that the in situ regeneration of anion and cation resins in the service vessel is not practical. Therefore, it is necessary to transfer the resins from the service vessel to a specially designed regeneration system. There are various designs of external regeneration systems currently in use. One design regenerates both the cation and anion resins in a single vessel. This type of system presents critical design problems to prevent the sodium hydroxide from contacting the cation resin and the sulphuric acid from contacting the anion resin. Because of this design problem and certain operational problems the single vessel regeneration system has not found wide acceptance.
Another design is a two vessel regeneration system in which the anion and cation resins are transferred into a separation/cation regeneration vessel. The resins are backwashed with water to expand the bed and classify the resins into an upper anion exchange resin layer and a lower cation exchange resin layer. The anion resin is then removed to an anion regeneration vessel where it is cleaned and regenerated. The cation resin is cleaned and regenerated in the separation/cation regeneration vessel. This design requires the complete separation of the anion exchange resin and the cation exchange resin. Various techniques have been used to effect such separation, including those disclosed in U.S. Pat. No. 3,385,787 to Crits et al, U.S. Pat. No. 3,429,807 to Burgess, U.S. Pat. No. 3,582,504 to Salem et al., U.S. Pat. No. 3,634,229 to Stanley Jr., U.S. Pat. No. 3,826,761 to Short, and U.S. Pat. No. 4,120,786 to Petersen et al. Although the above techniques have improved the degree of separation of the anion resin and the cation resin, they have not achieved complete separation. In practice, the consequence of imperfect separation is that a small proportion of the cation resin is inevitably saturated by the anion resin regenerant and conversely a small proportion of the anion resin is saturated by the cation resin regenerant. Both of which reduces the level of performance when the resins are returned to service.
In an effort to reduce the mixing of the anion resin and the cation resin at the interface between the resins after the backwash separation, it has been suggested to provide an intermediate layer of inert resin material of specific density intermediate the specific densities of the anion and cation resins. One example of such a system is disclosed in U.S. Pat. No. 2,666,741. The system disclosed in this patent hydraulically separates the resins in the service vessel into an upper anion resin layer, an intermediate inert resin layer and a lower cation resin layer. The anion resin and the cation resin are regenerated by passing sodium hydroxide regenerant into the inert layer and upwardly through the anion resin and passing acid regenerant into the inert layer and downwardly through the cation resin. Although this system provides advantages over other systems which regenerate in the service vessel, it has not solved many of the problems inherent in the regeneration of the anion resin and the cation resin in the service vessel. Also, the inert resin in the service vessel occupies space which can otherwise be occupied by additional anion and cation ion exchange resin. Accordingly, it is necessary to increase the size of the service vessel to make space for the inert resin.
The use of an intermediate density inert resin has also been heretofore disclosed in a two vessel regeneration system. Such a system is disclosed in U.S. Pat. Nos. 4,298,696 and 4,457,841. This system includes a separation/anion regeneration vessel and a cation regeneration vessel. The inert resin is mixed with the anion and cation resin in the service vessel. The resin from the service vessel is transferred into the separation/anion regeneration vessel wherein it is separated into an upper anion resin layer, an intermediate inert resin layer, and a lower cation resin layer. The cation resin layer is then hydraulically transferred to the cation regeneration vessel, leaving behind the anion resin and most of the inert resin. The anion resin is regenerated and rinsed in the separation/anion regeneration vessel and the cation resin is regenerated and rinsed in the cation regeneration vessel. The cation resin is then transferred back to the separation/anion regeneration vessel, wherein it is mixed with the anion resin and the inert resin and transferred back to a service vessel. This system also transfers the inert resin along with the anion and cation resin back into the service vessel and, thus, either results in reduced service capacity or requires an increase in the size of the service vessel. It should also be noted that the resin from each service vessel must include a quantity of inert resin. This system contemplates removal of any cation fines (heel) which are not separated out and transferred with the cation resin by the additional step of floating the anion resin in a saturated brine solution and removing the cation heel from the bottom of the separation/anion regeneration vessel.
A regeneration system has also been proposed in U.S. Pat. No. 4,388,417, assigned to the same assignee as the present invention, in which the inert resin remains in the regeneration system and is not returned to the service vessel. This system provides for the transfer of the exhausted anion and cation resins from the service vessel to a separation/anion regeneration vessel which contains a quantity of inert resin of a specific density intermediate to the specific densities of the anion and cation resins. Following a sequence of wash, drain, and air scrub steps, the resins are backwashed from a bottom distributor to classify the resins into an upper anion resin layer, an intermediate inert resin layer, and a lower cation resin layer. The cation resin layer is then transferred from the bottom of the separation/anion regeneration vessel into a cation regeneration vessel. Upon completion of the cation resin transfer, the separation/anion regeneration vessel is drained and caustic soda solution of a concentration in the range of 10-18% is cycled therethrough, causing the anion resin to float and any traces of cation resin and the inert material to sink to the bottom of the vessel, leaving a layer of caustic soda inbetween. The floating anion resin is then transferred from the separation/anion regeneration vessel to an anion rinse vessel wherein it is suitably rinsed and held. The inert resin and the cation heel are retained in the separation/anion regeneration vessel awaiting the delivery of the next exhausted resin charge. The cation resin in the cation regeneration vessel is regenerated with sulfuric acid and rinsed in a conventional manner. The anion resin is then transferred from the anion rinse vessel to the cation regeneration vessel wherein it is air mixed with the cation resin and final rinsed, whereupon the mixed anion and cation resin is held awaiting transfer to a service vessel.
A regeneration system is disclosed in U.S. Pat. No. 4,442,229, of the type disclosed in U.S. Pat. Nos. 4,298,696 and 4,457,841, wherein after separation and regeneration of the cation and anion resin, the resin in the separation vessel is further classified to cause contaminant cation resin to settle to the bottom of the vessel. The settled contaminant cation resin and some anion material is transferred from the bottom of the separation vessel through the cation transfer conduit into a separate isolation vessel. The regenerated cation resin is transferred back into the separation vessel and remixed with the regenerated anion resin, which mixture is then removed from the separation vessel. The materials from the isolation vessel are then transferred back into the separation vessel to await receipt of the next batch of mixed bed of resins to be regenerated. This patent also discloses the utilization of the cation transfer conduit to isolate the contaminant cation resin and some anion resin instead of an isolation vessel.